Definition
A diatonic substitution replaces a chord with another from the same scale that serves the same function and shares with it at least two notes (triads) or three notes (seventh chords). The melody remains compatible and the harmonic direction does not change, but the color does.
The functional families in C major
| Function | Chords | Common tones with the main chord |
|---|---|---|
| Tonic | Cmaj7, Em7, Am7 | Em7 has E G B; Am7 has C E G |
| Subdominant | Fmaj7, Dm7 | Dm7 has F A C |
| Dominant | G7, Bø7 | Bø7 has B D F |
In A minor: tonic Am7, Cmaj7 (relative); subdominant Dm7, Fmaj7, Bø7; dominant E7, G#°7.
Seventh chords a third apart
Every diatonic seventh chord shares three notes with the one built a third below and the one a third above:
- Cmaj7 (C E G B) and Am7 (A C E G): common C E G.
- Cmaj7 and Em7 (E G B D): common E G B.
- Dm7 (D F A C) and Fmaj7 (F A C E): common F A C.
- G7 (G B D F) and Bø7 (B D F A): common B D F.
This is why Em7 is "Cmaj9 without the root" and Am7 is "C6 with A in the bass."
Classic substitutions
- I → vi (deceptive cadence, or at the start of a turnaround): C Am Dm G in place of C C Dm G.
- I → iii: Em7 A7 Dm7 G7 in place of Cmaj7 A7 Dm7 G7 (the classic iii–VI–ii–V turnaround).
- IV → ii: Dm7 G7 C instead of F G C: the bass rises by a fourth (D → G), more "jazzy" voice leading.
- V → vii°: B° → C (leading-tone seventh chord as a dominant without root).
- iii in place of V (in the opposite direction, "weak dominant"): Em → Am in many pop songs where you don't want the strength of the dominant.
Checking the melody
The substitution is valid only if the melody note is a chord tone or an available tension:
- Melody C over Cmaj7 → Am7 (b3) ✓, Em7 (b6: avoid note) ✗.
- Melody B over Cmaj7 → Em7 (5) ✓, Am7 (9) ✓.
- Melody F over Dm7 → Fmaj7 (root) ✓; over G7 → Bø7 (diminished 5th) ✓.
Real example
The rhythm changes are played interchangeably with I–vi–ii–V (Bb G7 Cm7 F7) or iii–vi–ii–V (Dm7 G7 Cm7 F7): this is a diatonic substitution I → iii applied at the start of each turn. In pop, the difference between the I–vi–IV–V progression (1950s) and I–vi–ii–V (Blue Moon, Heart and Soul in its second form) is precisely IV → ii.
Exercise for the producer
- Take a I – IV – V – I progression in C.
- Write 4 versions: replace I with vi, then with iii; replace IV with ii; replace V with vii°/Bø7.
- Leave the melody unchanged and listen to which substitutions "work" and which create avoid notes.
- Choose the best version for the second verse: same song, new color.
Related topics
Diatonic functions, extended tonal functions (Riemann), substitution by thirds, reharmonization, chord-scale theory and avoid notes.